Transactinides

The term "transactinides" is used to refer to all elements beyond the actinides—that is, those elements with atomic numbers larger than 103. Lawrencium, with atomic number 103 and a full inner 5f electron shell, ends the actinide series. According to atomic relativistic calculations, the filling of the 6d electron shell takes place in the first nine of the transactinide elements (those with atomic numbers 104 through 112). They are placed as a 6d transition series under the 5d transition series in the Periodic Table of the chemical elements.^✶ As of 2003 the discoveries of elements 104 through 111 had been confirmed, and element 112 was reported but not yet confirmed. The date of discovery and the names and symbols approved for each of these elements by the International Union of Pure and Applied Chemistry (IUPAC) in August 1997 and in August 2003 are given in Table 1, as is the date of discovery of the element that has been reported but not yet named. The naming of element 112 awaits a final IUPAC decision that there has been sufficient confirmation of its discovery to ask the discoverers to propose a name and symbol for consideration.

^✶See Periodic Table in the For Your Reference section at the beginning of this volume.

All the known transactinides were first positively identified using physical rather than chemical techniques because of their very short half-lives

and small production rates. Production rates decrease from a few atoms per minute for element 104 to only a few atoms per week for element 111. Methods other than the classical chemical separation techniques for determining the atomic number (proton number) of a new element had to be developed. The production and study of both the chemical and nuclear -decay properties of the transactinides require special facilities and capabilities. These include preparation and handling of radioactive targets, access to an accelerator that can furnish high-intensity beams of the required light-to-heavy ions for irradiating the targets, and a method for rapidly transporting the desired short-lived isotopes to a suitable chemical or physical separation system. In the 1960s such facilities were available in Russia at the cyclotron at the Joint Institutes for Nuclear Research in Dubna and in the United States at the Heavy Ion Linear Accelerator (HILAC) at the Lawrence Berkeley Laboratory in California.

From the mid-1960s to the early 1970s, the Russian and U.S. groups reported conflicting claims concerning the discoveries of elements 104 and 105, primarily due to the difficulty in positively determining the atomic number of the short-lived isotopes involved. Different techniques were used by the groups to attempt to provide positive proof of the discoveries. TheU.S. group used an alpha-alpha correlation technique to identify the atomic number of alpha-decaying isotopes (alpha-decay is the emission of a ⁴He²⁺ nucleus) of elements 104 and 105 by observing the time correlations between alpha-particles emitted by the parent element and those of the already known element 102 (nobelium) or 103 (lawrencium) daughter activities. The Russian group relied primarily on the detection of spontaneous fission (SF) decay. Spontaneous fission is one of the modes of decay found in elements of higher atomic number than actinium; in this process the nucleus "spontaneously" splits into two large "fission fragments." Half-lives for SF decay range from microseconds to billions of years (e.g., the SF-decay branch in plutonium-244 has a half-life of nearly 100 billion years). Detection of SF fragments is a very sensitive method, but the atomic number and mass of the fissioning nucleus are effectively destroyed in the fission process, so it is extremely difficult to identify the fissioning element with certainty. In the case of element 105, the Dubna group also performed an alpha-alpha correlation experiment, but these experiments were rather inconclusive and did not agree with later confirmatory experiments. They also measured SF decay that they attributed to element 105 based on its half-life, proposed production reaction based on data from other experiments, and angular distribution of the products of the nuclear reaction. Chemical separations, even if rapid enough, are not definitive without some independent positive determination of atomic number, since the unknown chemistry of a new element cannot be used to prove its atomic number. TheU. S. and Russian groups proposed different names for these elements, and the controversy was not resolved until the compromise set of names shown in Table 1 was finally approved by IUPAC in 1997. Element 106 was produced and identified at the SuperHILAC by the Berkeley group in 1974 and confirmed by an independent group in 1994. The name seaborgium (Sg) in honor of Professor Glenn T. Seaborg was then proposed by the discovery group. After initially being rejected because Seaborg was still alive, the name was approved.

In the 1970s the Separator for Heavy Ion Reactions (SHIP) was constructed by a group at the Universal Linear Accelerator (UNILAC) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany. The researchers hoped that SHIP might take them to the long-sought island of SuperHeavy Elements (SHEs) in the predicted region of extra stability around the "magic" numbers of 112 to 114 protons and 184 neutrons. SHIP was designed to separate the "slow" fusion products created in the nuclear reactions from the "fast" heavy-ion beams that produced them by using a combination of electric and magnetic fields. The reaction products were then implanted in a detector system some distance away from the target. The time, energy, and position of the implant and all subsequent decays and other information were recorded by a computer system. The original event was then correlated unambiguously with its decay to already known properties of many generations of known daughter isotopes, and its atomic number and mass were unequivocally determined.

Discovery of elements 107 through 109 was reported by the SHIP group between 1981 and 1984. After some improvements were made in SHIP, elements 110 through 112 were produced and identified in 1995 and 1996. However, the most neutron-rich of the isotopes first reported for elements 110 through 112 contain only 165 neutrons or fewer, far from the predicted SHE region. Their half-lives are milliseconds or shorter, and most scientists do not consider them to be SHEs. Even though the attempt to produce element 113 was unsuccessful, these results gave researchers new hope that the SHEs might yet be reached, sparking a renaissance of interest in their production. Between 2000 and 2002, the production of element 114 with 174 neutrons and a half-life of about 3 seconds and element 116 with 176 neutrons and a half-life of 50 milliseconds was reported by a Dubna/LLNL collaboration using the on-line separator at the Dubna cyclotron. The group used beams of the heaviest stable calcium isotope (mass 48) and either ²⁴⁴Pu or ²⁴⁸Cm targets, to make element 114 or 116, respectively. They have proposed that these should be called SHEs, although they are still far from the 184-neutron shell. The element 112 and 110 daughters resulting from the element 116 and 114 alpha-decay chains were reported to have half-lives on the order of 10 seconds. It is extremely important

to confirm these results, as investigations of their chemical properties could then be considered.

Chemical Studies

A primary goal of the chemical studies of the transactinides is to determine their placement in the Periodic Table by comparing their properties with those of their lighter homologues and with theoretical predictions. These studies seek to probe the uppermost region of the Periodic Table, where the influence of relativistic effects on chemical properties should be strongest and where deviations from simple extrapolation of known trends within chemical groups in the Periodic Table have been observed. There are many challenges for chemical studies in addition to those already outlined for physical separation techniques: An isotope with a half-life of at least a second must be known. Its decay characteristics must be well established and must furnish a positive "signature" so that it can be shown that the desired element is actually being studied. Again, the method of measuring time-correlated "mother-daughter" alpha-decay chains is most definitive. Because the production rates are so low, the results of many separate identical experiments must often be combined. Very efficient separation methods must be devised that reach equilibrium rapidly and can be conducted in a length of time that is short compared to the half-life. These separations should give the same results on an "atom-at-a-time" basis as they would for the usual macro-scale laboratory experiments conducted with milligram quantities. Both aqueous- and gas-phase chromatographic methods have been shown to meet these criteria. A rapid and efficient method for transporting these atoms from the production site to a suitable chemical separation system is needed. Computer-controlled automated systems have been developed for these studies.

Typically, the isotope with the longest half-life and largest production rate is chosen. This is not necessarily the first isotope discovered. The half-lives of the isotopes used in the first definitive chemical studies of the transactinides were: 75-second ²⁶¹Rf; 34-second ²⁶²Db; 21-second ²⁶⁶Sg; 17-second ²⁶⁷Bh; and approximately 14-second ²⁶⁹Hs. Their well-known alpha-decay properties were used for positive identification. As of 2003, 0.04-second ²⁶⁸Mt was the longest-known isotope of Mt, and studies of its chemical properties awaited discovery of a longer-lived isotope.

From 1969 to 1976 researchers at Dubna reported studies of the volatilities of the halides of elements 104 and 105, suggesting that the halides of these elements behaved similarly to those of their lighter group-4 and group-5 homologues. Because only spontaneous fission was detected, the identity of the species being measured could not be positively attributed to element 104 or 105.

The first study of the solution chemistry of Rf was performed at Berkeley in 1970 and showed that Rf had a stable tetravalent state with properties similar to the group-4 elements Zr and Hf and different from Lr and the other trivalent actinides. This established that Rf should be placed in the Periodic Table as the heaviest member of group 4 and the first member of a new 6d transition series. It also confirmed the 1945 prediction of Seaborg that the actinide series should end with element 103. The first studies of the solution chemistry of element 105, conducted at the 88-Inch Cyclotron at Berkeley, were reported in 1988 and showed that the element behaved similarly to the group-5 elements Nb and Ta in its sorption properties, but differently from the group-4 elements. However, in extractions into certain organic solvents, Db(Ha) and Ta extracted but Nb did not, creating a renaissance of interest in more detailed studies of the behavior of element 105.

Subsequently, extensive studies of both the aqueous and gas-phase chemistry of elements 104 and 105 were conducted using manual and sophisticated computer-controlled automated systems. Although in general these studies confirmed that the elements' chemical properties are similar to those of the group-4 and group-5 elements, respectively, unexpected deviations from simple extrapolation of known trends within the groups were found. Theoretical investigations based on molecular relativistic calculations can explain these results and provide guidance for experimentalists in designing the sophisticated and resource-intensive experiments required for future experiments. With this guidance and further improvement in experimental techniques, chemical studies were extended to Sg, Bh, and Hs. Aqueous- and gas-phase studies of Sg were conducted between 1995 and 1998 by an international team of researchers working at the UNILAC at GSI. Gas-phase studies of an oxychloride of Bh were reported in 2000, and separation of Hs as a volatile oxide similar to that of osmium tetroxide was reported in 2002. It appears that all of these elements should be placed in the Periodic Table as members of the 6d transition series under groups 4 through 8 of the Periodic Table and that the trends in properties within the groups can be described by relativistic calculations. No investigations of the solution chemistry of Bh and Hs has been conducted as of mid-2003 because the aqueous chemistry and preparation of samples suitable for measuring alpha-particles or fission fragments is too slow. Very fast liquid-liquid extraction systems followed by direct incorporation of the activity in a flowing liquid scintillation detection system have been used for elements 104 and 105 and may prove applicable.

The improvement in experimental techniques for atom-at-a-time studies of elements with both short half-lives and small production rates has permitted chemical studies of both aqueous- and gas-phase chemistry of the transactinides through Sg, and gas-phase studies of Bh and Hs have been conducted. Extending the solution-chemistry studies will depend on the development of faster systems. Studies of elements beyond Hs await discovery of longer-lived isotopes with definitive decay properties and production rates that are larger than a few atoms per week. If isotopes with half-lives of years or more are discovered, then techniques for "stockpiling" them might be envisioned. The synergistic interactions between theory and experiment are leading to a better understanding of the chemistry of the transactinides and the role and magnitude of relativistic effects.

see also Actinides; Actinium; Americium; Berkelium; Californium; Einsteinium; Fermium; Lawrencium; Mendelevium; Neptunium; Nobelium; Nuclear Chemistry; Nuclear Fission; Plutonium; Protactinium; Radioactivity; Rutherfordium; Seaborg, Glenn Theodore; Thorium; Transmutation; Uranium.

Darleane C. Hoffman

Bibliography

Harvey, Bernard G., et al. (1976). "Criteria for the Discovery of Chemical Elements." Science 193:1,271–1,272.

Hoffman, Darleane C. (1994). "The Heaviest Elements." Chemical & Engineering News 72: 24–34.

Hoffman, Darleane C.; Ghiorso, Albert; and Seaborg, Glenn T. (2000). The Transuranium People: The Inside Story. London: Imperial College Press.

Hoffman, Darleane C., and Lee, Diana M. (1999). "Chemistry of the Heaviest Elements: One Atom at a Time." Journal of Chemical Education 76: 331–347.

Seaborg, Glenn T. (1945). "The Chemical and Radioactive Properties of the Heavy Elements." Chemical & Engineering News 23: 2190–2193.

Seaborg, Glenn T. (1985). "The Transuranium Elements." Journal of Chemical Education 62: 463.

Seaborg, Glenn T., and Loveland, Walter D. (1990). The Elements Beyond Uranium. New York: John Wiley & Sons.

Internet Resources

International Union of Pure and Applied Chemistry, Inorganic Chemistry Division. Available from <http://www.iupac.org/news/archives/2003/naming110.html>.